How do you have a relationship

Buddhism has made me realise that everything is impermanent and undergoing the process of destruction including intimate relationships however if I know this then why is it still so painful when it happens? And what is the point of trying to build a life together with another person when it's inevitably going to end? Sometimes it all feels like such a cruel joke. I was in a relationship for 15 years and never thought it would end but it did and 6 months later I still feel so sad. I don't want to ever get involved intimately with another person ever again because I don't ever want to go through that pain again. Yes this may be aversion to pain but why put yourself through that if you can avoid it? Sure there will be more pain from other things but the pain of separation from a loved one feels worse than a death. It actually feels like I could die.